DAMASCUS, Syria (AP) — A roadside bomb struck a bus in Syria's central province on Thursday, killing 19 people, a local government official said.

The explosion in the village of Jbourin also wounded four people on the bus, according to the official from the governor's office in Homs province who spoke on condition of anonymity in line with regulations.

The village is predominantly Alawite, an offshoot of Shiite Islam and a minority sect to which President Bashar Assad belongs, but it also has Christians and Sunni Muslims.

It was not immediately clear why the bus was targeted but Syria's civil war, which has left more than 100,000 dead since the crisis erupted in March 2011, has taken increasingly sectarian overtones. Most of the rebels trying to overthrow Assad belong to the majority Sunni sect.

Elsewhere in Syria, al-Qaeda-linked gunmen captured a town near the Turkish border after heavy clashes with a rebel group that held the area, an activist group said Thursday.

It was the latest development in what has been a relatively new component in the conflict — stepped-up infighting between extremists with ties to al-Qaeda and Western-backed opposition groups.

The U.S. and its European and Gulf allies are increasingly concerned about the rising prominence of Islamists among the rebels, who have been playing a major role in the battles against Assad's forces.

The Britain-based Syrian Observatory for Human Rights, an activist group that monitors the violence, said members of the al-Qaeda offshoot known as the Islamic State in Iraq and the Levant stormed the town of Azaz in the northern Aleppo province on Wednesday evening, forcing the opposition fighters from the Western backed bloc to pull out.

There has also been infighting among rebel groups in the eastern province of Deir el-Zour, which borders Iraq, and in the north where al-Qaeda fighters from the ISIL and their allies in the Nusra Front have been battling Kurdish anti-government rebels for months. The infighting has left hundreds dead.

The fighting in Azaz broke out on Wednesday, when ISIL fighters tried to detain a German doctor they accused of taking pictures of their positions on behalf of their rivals, said Rami Abdul-Rahman, who heads the Observatory. The doctor, who was a volunteer in the region, escaped but the two rebel factions started fighting.

Amateur videos showed dozens of gunmen with heavy machine guns mounted on pickup trucks gathering at the nearby border crossing with Turkey. The videos appeared genuine and corresponded to other Associated Press reporting on the events depicted.

Abdul-Rahman said three opposition fighters and two jihadis were killed in the fighting. On Thursday, mediation was under way to get the jihadis to leave Azaz, he said.

Also Thursday, the international aid agency Oxfam issued an appeal, saying many donor countries are failing to provide their share of the urgently-needed funding for the humanitarian response to Syria crisis. Oxfam said donors, including France, Qatar, United Arab Emirates and Russia, should prioritize funding the U.N.'s $5 billion appeals.

Oxfam's report came ahead of next week's donors meeting in New York. The donor countries have been influential in shaping the international response to the conflict, but should also bear their fair share of the burden of humanitarian aid, the agency said.

"Too many donor countries are not delivering the level of funds that is expected of them," said Colette Fearon, head of Oxfam's Syria program. "While economic times are tough, we are facing the largest man-made humanitarian disaster in two decades and we have to seriously address it."

"The scale of this crisis is unprecedented and some countries must start to show their concerns to the crisis in Syria by putting their hands in their pockets," Fearon said.

The fighting in Syria has forced 7 million people to flee their homes. Five million Syrians have been displaced inside the country and more than 2 million have sought refuge in the neighboring countries of Jordan, Lebanon, Turkey and Iraq, according to the U.N.